Cars and Trucks

Harry and I were reading one of his books for the 116th time last week...Here's how it went:
ME: "Apple. Alligator."
Harry turns the page.
ME: "Baby. Ball."
Harry turns the page.
ME: "Cat. Car."
Little Voice: "Cah..."
ME: "Cup." (WAIT WHAT JUST HAPPENED? IT'S PROBABLY JUST A FLUKE.)
ME: "Car."
Little Voice: "Cah..."
ME: "Car."
Little Voice: "Cah."
ME: "OMIGOD! YOU UNDERSTAND! YOU SAID A WORD! AHHHHHHHHH!"
Little Voice is now scared to death and will probably never say another word.

We move onto another book. We get to Harry's favorite page of "8 big trucks." I point out "trucks." He sticks his arms straight out, shakes and seems to be using all of his might to let out this sound: "Truh...." "TRUUUHHHH...."
I am trying not to scare him, but wondering why all of a sudden TODAY he is suddenly clued into the English language. We move onto other books, and he points out more "Truhs..."

I wonder why he picked those words?